The invention relates to a metal halide lamp intended to be operated on an electronic ballast, which lamp comprises a discharge vessel having a ceramic wall enclosing a discharge space which contains an ionizable filling comprising, in addition to Hg, a quantity of Na halide, two electrodes with tips being arranged at a mutual distance EA, and the discharge vessel having an internal diameter Di at least through the distance EA.
A lamp of this type is described in U.S. Pat. No. 5,923,127. The known lamp, which has eminent color properties (inter alia, general color rendering index Ra.gtoreq.80 and a color temperature T.sub.c of 3000 K), is integrated with the electronic ballast in the form of a switched-mode power supply (smps) and is thus very suitable as a light source for, inter alia, interior lighting. This lamp is based on the recognition that a good color rendition is possible when Na halide is used as a filling constituent of a lamp and a strong widening and reversal of the Na emission in the Na-D lines occurs during lamp operation. This requires a high temperature of, for example, 1170 K (900.degree. C.) of the coldest spot T.sub.kp in the discharge vessel. When reversing and widening the Na-D lines, these take the shape of an emission band in the spectrum with two maxima at a mutual distance .DELTA..lambda..
The requirement for a high value of T.sub.kp results in a relatively small discharge vessel, which, in the practical lamp, leads to a wall load of 70 W/cm.sup.2 measured across the internal surface area of the cylindrical part of the discharge vessel through the distance EA. The required high temperature precludes the use of quartz or quartz glass for the wall of the discharge vessel and necessitates the use of ceramic material for the wall of the discharge vessel.
The ceramic wall in this description and claims is understood to mean both a wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al.sub.2 O.sub.3, or metal nitride, for example AlN.
The electronic ballast comprises a high-frequency converter which converts, as smps, the low-frequency power supply of the mains into a high-frequency current through the lamp. In this case, it should be ensured that the high frequency is chosen to be such that it does not give rise to acoustic resonance phenomena in the lamp. Another, generally used configuration as an smps for high-pressure discharge lamps consists of a concatenation of rectifier means, a preconditioner, a converter and a commutator to which the lamp is connected. The preconditioner is used for generating a DC voltage for power supply of the converter while withdrawing a current which is sinusoidal in a satisfactory approximation from the mains operating as the power supply source. The commutator provides for an, often low-frequency, AC current through the lamp. Both forms of the electronic ballast are designed in such a way that the voltage across the lamp is approximately 90 V in the nominal operating condition of the connected lamp. It is thereby achieved that the relevant electronic ballast is suitable for operating known lamps which are generally designed for operation at a lamp voltage of approximately 90 V and can be operated on a ballast in the form of a ballast coil.
In addition to Na, the filling of the discharge vessel may comprise Tl and/or one of the rare earth metals, with which a desired value for the general color rendering index Ra.gtoreq.80 and the color temperature T.sub.c between 2700 K and more than 4200 K is realized. In this description and claims, the elements Y and the lanthanides are considered as rare earth metals. Due to the formation of compounds with O.sub.2 in ceramic discharge vessels based on metal oxide, Sc is not suitable as a filling constituent.
A drawback of the known lamp is that it has a relatively low specific light output. A further drawback of the known lamp is that, also as a result of the relatively small dimensions of the discharge vessel, a relatively rapid blackening of the wall of the discharge vessel occurs, inter alia, due to deposition of evaporated material on the wall of the electrodes, so that the lumen maintenance and hence the practical lifetime of the lamp is influenced very detrimentally.